In the oil field industry, as well as others, electrical information is often transmitted through a transmission channel which consists of a long cable. In oil field applications, the long cable may extend from the bottom of a well to the earth's surface, and the electrical information may represent logging data collected at the well bottom. Increased sophistication of well logging tools makes the gathering of increasingly detailed down-hole information more practical. However, a typical logging cable presents a severe bottleneck to the communication of such information to the earth's surface. A typical logging cable exhibits a usable signal bandwidth extending from DC to approximately only 30 KHz at best. Thus, when higher frequency analog waveforms are transmitted to the earth's surface over a logging cable, amplitude and phase distortions caused by the logging cable unacceptably corrupt the transmitted signal, especially the signal's high frequency details.
While amplitude and phase distortions may be compensated to a degree, the compensation techniques are generally tailored to specific cable characteristics. Consequently, compensation techniques which permit the transmission of high frequency signals through a particular cable, of a particular length, used in a particular environment, fail to adequately compensate for distortions caused by other cables, lengths, or environments.
Frequency domain techniques may be used to gain independence from cable specifics. Thus, by translating the transmitted information into frequency fluctuations, amplitude and phase distortions do not harm the frequency encoded information, and are therefore easily compensated. Moreover, such compensation utilizes conventional techniques which need not account for individual cable parameters.
On the other hand, frequency domain techniques, such as conventional frequency modulation (FM), require a suitable demodulator at the receiving end of the cable to translate communicated information back into its original form. Moreover, conventional FM demodulators, which include phase locked loop circuits and integrators driven by zero-crossing detectors, fail to provide an adequate solution to FM demodulation in this environment.
Conventional FM demodulators from commercial applications, such as aircraft telemetry, typically allow an information signal's bandwidth to exhibit a maximum frequency deviation from a carrier frequency, typically .+-.15% to .+-.30% of the carrier's center frequency. For many applications, such as when public airwaves are used to broadcast information, regulatory requirements limit frequency channels to prevent inter-channel interference, resulting in what is known as narrowband FM. However, when a private cable serves as a transmission channel, large frequency channel bandwidths relative to the carrier center frequency could be used if a satisfactory demodulator were available. Thus, conventional demodulator techniques limit available channel bandwidth for reasons which are not pertinent to well logging and other applications utilizing long electrical cables.
An integrator driven by a zero-crossing detector and a pulse generator represents a popular, conventional demodulation technique which suffers from yet another problem. Specifically, the current transients produced by zero-crossing detectors and pulse generators tend to inject an excessive amount of noise into surrounding noise-sensitive circuits. As a result, highly accurate signal reproductions cannot be achieved.
Still further, conventional FM demodulators, which tend to be predominantly analog in design, are often excessively difficult to utilize due to a requirement for numerous adjustments and calibrations. Consequently, such demodulators produce potentially inaccurate results when such adjustments and calibrations are either omitted or performed incorrectly. Oftentimes, critical circuits are temperature sensitive. Moreover, such conventional FM demodulators utilize essentially analog processes to recover the modulating signal from a carrier. When these conventional FM demodulators are embedded in a digital system, the recovered modulating signal must be digitized for processing by a computer or other digital circuits. Thus, digitizing errors compound reconstruction errors to cause such conventional FM demodulators to generate unacceptably large overall errors.